This invention is related generally to an improved method and apparatus for storing and retrieving digital data utilizing holograms and more specifically, to a novel and improved method and apparatus for sequentially recording and retrieving individual bits of digital data in and from such holographic memories.
The use of holograms as a medium for storing digital data is well known in the art. A conventional holographic data memory is formed by initially arranging the data to be stored in a planar array. This array is composed initially as a "page" of data and is placed in an intermediate temporary storage device commonly referred to as a "page composer". The page composer is usually an array of cells, each of which may be made either opaque or transparent according to whether a binary 0 or 1 is to be stored at that particular cell address. After the entire page of data has been formed, it is illuminated with laser light and holographically recorded on a holographic storage medium.
Several formats for recording a large number of such pages in a single hologram memory have been reported. In one such form, a page array of small normally transparent cells is composed. This page is then recorded holographically on a photographic medium as a single hologram by illuminating the transparent page composer with laser light and focussing the transmitted laser light onto a small area of the recording medium where it is caused to interfere with a reference beam. Following the recording of the first page, another page of digital data is composed in the page composer, and likewise recorded by interfering with the reference beam but in another area of the recording medium. This process is continued until either all of the data is recorded or space in the recording medium is exhausted.
Another format, which is less commonly employed, involves the use of the angular selectivity inherent in the recording of so-called Lippmann-Bragg volume holograms. Such holograms are formed throughout the volume of a thick recording medium instead of on the surface of a thin planar recording medium. In a volume holographic recording, each page of data is superimposed upon a number of others in the same volume of the recording medium; however, with each exposure, the reference beam is incident on the medium from a different recording angle.
During the playback or read-out of data from holograms formed according to the former methods, a reference laser beam is employed. In the previously described first format, the reference beam is directed onto a selected small area including the desired hologram with its page array of data. An image of the original page composer with its array of digital data in the form of light and dark spots for that page is reconstructed. The reconstructed page image is formed in a detection plane where an array of photodetectors is placed to interrogate each individual data bit.
The projected data array is read-out electronically with the photodetectors which sense the presence or absence of light at each bit position in the imaged array. The read-out of a page from a Lippmann-Bragg volume holographic memory is performed in a similar manner. A reference beam is directed into the volume hologram from the specific angle used to record that particular page and the resulting data array is imaged and reconstructed for electronic detection. Other pages in the volume hologram memory are accessed by orienting the reference beam at different angles associated with those pages during recording.
These prior art methods for storing and retrieving data present certain disadvantages. For example, to be practical, a page of data must include a large number of individual bits of digital data. The parallel recording of an entire such page of digital data involves the simultaneous illumination of all of the data bits in the page composer with a common data beam. Such simultaneous illumination creates intermodulation noise which is caused by the interference of rays from individual data bits at the recording medium. During read-out, such noise results in a flare of diffracted light for which special care must be taken to keep the flare from creating detection interference on the detector plane. Failure to take such precautions causes serious signal-to-noise problems. In order to avoid flare problems, the readout involves using reference beams at relatively large angles from the data beams to prevent the flare from falling directly or being scattered onto the detector plane. Since no data bits can be recorded within a minimum solid angle around the reference beam where the flare is located, that minimum part of the storage capacity of the holographic recording medium around the reference beam is essentially wasted.
Another disadvantage of the prior art systems involves the requirement that a full page of data must be temporarily stored to enable an optical version to be composed followed by subsequent storage on the holographic recording medium.
Another disadvantage arises because a substantial amount of laser beam power is wasted through losses encountered with the page composer which is opaque in many places such as the areas where zeroes are stored, the areas between the data, unused addresses and border areas. Consequently, much of the laser light used to illuminate the page composer is not transmitted and recorded, but is wasted by absorption. Hence, for any given laser beam power that may be available, the time to store data will be significantly longer than in comparison with a storage method with which all of the laser power could be used to record the data. The only manner in which storage time can be reduced is to increase the laser illumination power. However, given the present state of the art in lasers, such an increase can only be accomplished at significant increases in cost, both for operation and equipment.
The page composer required in prior art holographic data memory devices involves a complex structure utilizing many individually addressable electro-optic light valve cells, commonly of the order of 10,000 or more, to achieve practical densities in the storage medium. Such a page composer is extremely complex to design and to produce; it thus represents a generally undesirable component in a hologram memory storage system.
A further disadvantage occurs during read-out and has two aspects. This disadvantage is that the entire page of digital data is presented simultaneously to the read-out. If, for example, a page of data is a matrix of 10,000 bits, a like number of photodetectors and their associated circuitry are required. Obviously, such a device is complex and costly in design and construction and low in reliability. Furthermore, unless the read-out is directly integrated into a computer main frame, the data cannot be utilized at a speed even close to that at which it is available. Normally, the data is utilized in a sequential manner. Also, in many instances, only one or a few bits of data from a page are actually required at a specific time. At such times, generation of all data stored in that page creates much waste.